Browsing by Subject "Fine structure"

Now showing 1 - 4 of 4
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    A fine structural review on the spermatozoa of Cyprinidae with attention to their phylogenetic implications
    (Murcia : F. Hernández, 2009) Fürböck, Sonja; Lahnsteiner, Franz; Patzner, Robert A.
    The fine structural organization and dimensions of spermatozoa from species of 4 subfamilies of the Cyprinidae (Barbus barbus, Carassius carassius, Cyprinus carpio carpio, Cyprinus carpio haematopterus, Abramis brama, Alburnoides bipunctatus, Alburnus alburnus, Chalcalburnus chalcoides mento, Chondrostoma nasus, Hypophthalmichthys molitrix, Leuciscus cephalus, Phoxinus phoxinus, Rutilus rutilus, Rutilus meidingerii, Scardinius erythrophthalmus, Vimba vimba and Ctenopharyngodon idella) are compared with each other as well as with results from other studies. Based on these descriptions it is investigated whether sperm structure reveals correlations with the existing systematics and if it could be a useful taxonomical parameter. The scatter plots based on the discriminate analysis and the neighbour-joining trees based on a Mahalanobis distance matrix reveal that sperm organization is related with systematics in many aspects. However, in some cases there are also clear differences between relations found on the basis of sperm morphology and between the systematic relations.
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    Fine structure of the tapetum lucidum in the short-tailed stingray Dasyatis brevicaudata
    (Murcia : F. Hernández, 1994) Braekevelt, Charlie R.
    The tapetum lucidum of the short-tailed stingray (Dasatis brevicaudata) is located in the choroid of the superior fundus immediately external to the choriocapillaris. In this species the tapetum consists of a single layer of overlapping cells oriented at an angle of about 30" to the incoming light. These tapetal cells alternate with and are separated from one another by melanocytes which extend beyond the tapetal cells to intervene between the tapetal cells and the choriocapillaris. The tapetal cells and the melanocytes are flattened plate-like cells with their widest dimension facing the retina. Internally the tapetal cells display a peripherally located vesicular nucleus with most organelles in a paranuclear location. The bulk of the cell is packed with regularly spaced crystals reported to be guanine. The size and spacing of these reflective crystals is commensurate with constructive interference. In lightadaptation, the melanosomes of the intervening melanocytes are widely dispersed and for the most part block the passage of light to the tapetal cells. Although dark-adapted specimens were not examined, it seems reasonable to assume that in dark-adaptation, the melanosomes will retreat to unmask the tapetum and allow it to function as a reflective layer.
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    Retinal photoreceptor fine structure in the short-tailed stingray. Dasyatis brevicaudata
    (Murcia : F. Hernández, 1994) Braekevelt, Charlie R.
    The fine structure of the retinal photoreceptors has been studied by light and electron microscopy in the short-tailed stingray (Dasyatis brevicaudata). The duplex retina of this elasmobranch contains rods and cones in a ratio of about 10:l. No multiple receptors were noted nor was a repeating or mosaic arrangenient of the cones obvious. Only lightadapted specimens were studied but retinomotor movements of the photoreceptors were felt to be minimal or absent. The rods are large cylindrical cells with inner and outer segments of much the same diameter. Cones are short stout cells with a conical outer segment and a wider inner segment. Rod outer segment discs show peripheral incisures while cones do not. The inner segment of rods and cones are rich in organelles indicating much synthetic activity. The nuclei of rods and cones appear quite similar but cone nuclei are invariably at least partially protruded through the external limiting membrane which is formed by a series of zonulae adherentes between photoreceptor cells and Miiller cells. The synaptic region of both rods and cones display both invaginated (ribbon) synapses and superficial (conventional) synapses.
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    Retinal pigment epithelial fine structure in the short-tailed stingray. Dasyatis brevicaudata
    (Murcia : F. Hernández, 1994) Braekevelt, Charlie R.
    The retinal pigment epithelium (RPE), choriocapillaris and Bruch's membrane (complexus basalis) have been studied by light and electron microscopy in the short-tailed stingray (Dasyatis brevicaudata). The RPE consists of a single layer of cuboidal cells which display numerous basal (choroidal) infoldings as well as many apical (vitreal) processes which interdigitate with photoreceptor outer segments. The lateral cell borders are relatively smooth and joined by a series of tight junctions. Within these epithelial cells, smooth endoplasmic reticulum is the dominant organelle with only scattered profiles of rough endoplasmic reticulum. Polysosomes, mitochondria and phagosomes are abundant. Melanosomes are totally absent over the tapetum and are scarce in non-tapetal locations. The RPE nucleus is large, vesicular and centrally located. Rruch's membrane is a pentalaminate structure. The choriocapillaris is a single layer of large capillaries. The endothelium of these capillaries is typically thin but only minimally fenestrated.